Stuffing boxes for the purposes described are provided to allow relative movement of the spindle and the housing while maintaining an effective seal therebetween.
The seal within the chamber, also referred to as a packing, generally comprises a plurality of packing rings which, upon the application of axial pressure, bear radially upon the spindle and the housing wall defining the chamber to maintain sealing effectiveness in spite of the movement of the spindle.
The rings can include a base ring and a cover ring flanking a coiled graphite pressed ring.
It will be understood that the gland in the mounted state of the assembly applies pressure in the axial direction to the packing rings which is sufficiently strong to cause the packing rings to bear against the movable shaft or stem and to sealingly lie against the housing. A coiled graphite pressed ring will be understood to refer to a pressed packing ring of special construction. It can comprise a strip of expanded graphite with a metal strip inserted between the turns of the spiral.
This composite ring structure when inserted in the housing chamber, can be compressed in the axial direction to deform the strips in accordance with statistical laws into a more or less corrugated or zigzag conformation as seen in section in a plane parallel to the axis of the assembly.
Packings of this type have been found to be suitable for the valve spindles of high pressure valves and, especially high pressure steam valves. However, some leakage can occur when the intrinsic strength of the material of the coiled graphite pressed ring is less than the stresses which arise on actuation of the valve spindle because of adhesion forces transmitted to this ring.
Such adhesion forces depend upon the surface qualities of the valve spindle and can be very pronounced when the valve spindle is only actuated from time to time or when the surface quality of the valve spindle does not meet specifications. It is possible in such cases, upon movement of the valve spindle that pieces of the coiled graphite pressed ring will be torn away from it and bring about leakage or the like. Indeed, in high pressure applications, such packings under those conditions may "blowout". This cannot be prevented by braided or other conventional cover or base rings.